As a conventional apparatus for printing on both surfaces of a web of paper for rotary printing with a digital printer, there exist an apparatus using two digital printers in which after an upper surface of the web of paper fed from a paper supply unit is printed on by a first digital printer, the web of paper is turned upside down and an under surface of the web of paper is printed on by a second digital printer (see JP 2010-99955 A), and alternatively, as shown in JP 2005-335145 A, an apparatus using a single digital printer in which after a web of paper is fed to travel through the digital printer to print on an upper surface thereof, the web of paper is turned upside down, and the web of paper is fed again to travel through the digital printer to print on an under surface of the web of paper.
In the former of the prior art, there is the problem that two expensive digital printers are needed for upper and under surfaces, making the installation costly and the apparatus large-sized. Such a printing apparatus of duplex printing specification also gives rise to the problem that, in producing a number of single-side printed products, only one of the printers is worked, making the equipment inefficient.
In the latter of the prior art in which the single digital printer is used to accomplish printing on both surfaces one by one, it is required that the web of paper first supplied from the paper supply unit, printed on one surface and taken up onto a paper takeup unit should be removed from its takeup stand and be carried back into the paper supply unit. In the paper supply unit, the web of paper needs to be reset on its paper supply stand so that it can be fed out to cause the other (unprinted) surface to face a printing head of the ink jet printer and thereby to be printed on later. In this case, where for reasons of printing data it is required to start later printing from an area on the other surface opposite to a first image printed, normally the web of paper is rewound onto a rewinding machine on offline. In the absence of the rewinding machine, the web of paper having been printed on before, is removed from the takeup stand, is carried again to the paper supply unit in which it is set on the paper supply stand, without printing on, is rewound onto the paper takeup unit, so that the first print by the precedent printing lies on the outermost of the roll, and is thereafter removed from the takeup stand and carried again to the paper supply unit which it is set on the paper supply stand, necessarily for later printing.
In the latter of the prior art, therefore, in which duplex printing is effected in an apparatus of single-side printing specification, the problems have arisen that it entails heavy burdens on carrying single-side printed rolled paper for web resetting, on working operations such as those for conveying rolled paper to make use of a rewinding machine and on equipment and working operations to secure safety in handling weighty materials and components such as mounting and dismounting rolled paper.
In view of problems as mentioned above, it is an object of the present invention to provide a duplex printing apparatus that is designed and adapted to efficiently print on both surfaces of a web of paper using a single digital printer.